1. Field of the Invention
The present invention relates to a scanning electron microscope and, more particularly, to a thermal field emission electron gun comprising an emitter tip that is heated and applied with an electric field to eject electrons from the tip.
2. Description of the Prior Art
A thermal field emission electron gun comprising an emitter tip that is heated and applied with an electric field to eject electrons from the tip is used as an electron gun in a scanning electron microscope or other similar instrument. In the 1970s, an electron gun using a Schottky type emitter was developed as this kind of electron gun. This type of electron gun is described in detail in J. Vac. Sci. Technol., 16, p. 1704 (1979).
The emitter of the Schottky type electron gun is shown in FIG. 1, where to pins 2 and 3 are attached to a ceramic insulator disk 1. A tungsten wire 4 bent into a hairpin is stretched between the two pins 2 and 3. A cylindrical tungsten tip 5 of a single crystal is welded to the protruding front end of the tungsten wire 4. Usually, this tungsten tip 5 consists of a (100) single crystal and has a diameter of about 125 .mu.m.
This tungsten tip 5 is obtained by welding the single-crystal tungsten to the tungsten wire 4 in the form of a hairpin and then electrolytically polishing the front end of the single-crystal tungsten into a needle-like form. This needle-like tip makes an angle of approximately 10 to 30.degree..
After the tungsten tip 5 is machined as described above, it is coated with zirconium hydride (ZrH.sub.2), 6, and sintered. The zirconium hydride coating 6 is oxidized by the heating and becomes a mass of zirconium (Zr) or zirconium oxide (ZrO.sub.2), which is essential to forming a monolayer of zirconium at the tip of the single-crystal tungsten. The (100) plane of the thermal field emission electron gun with such a coating has a decreased work function and so the gun exhibits high brightness and long life.
The Schottky type emitter is fabricated by the processing described above. The principle of operation of the electron gun using this emitter is described by referring to FIG. 2. A heating power supply 7 is mounted between the two pins 2 and 3, which in turn are affixed to the insulator disk 1. This power supply 7 electrically energizes the tungsten wire 4 via the pins 2 and 3.
A suppressor electrode 8 is mounted to the insulator disk 1 so as to cover the disk 1 except for the front end of the tungsten tip 5. A suppressor power supply 9 applies a negative potential to the suppressor electrode 8 with respect to the tungsten tip 5. An extraction electrode 10 is mounted close to the front end of the tungsten tip 5. An extraction voltage is applied to the extraction electrode 10 from an extraction voltage source 11. An accelerating electrode (not shown) is positioned in front of the extraction electrode 10 such that an accelerating voltages is applied between the tungsten tip 5 and the accelerating electrode.
In the structure described above, a heating current is supplied to the tungsten wire 4 from the heating power supply 7 to heat the tungsten wire 4 up to approximately 1800 K. The extraction voltage, normally about 1 to 6 kV, is applied between the tungsten tip 5 and the extraction electrode 10 from the extraction voltage source 11.
As a result, electrons are extracted from the front end of the tungsten tip 5, accelerated to 3 kV, for example, by the accelerating electrode (not shown), and sharply focused onto a specimen by condenser lenses and an objective lens. The suppressor electrode 8 is put at a negative potential with respect to the tungsten tip 5 by the application of voltage from the suppressor power supply 9. Consequently, the thermal emission from the tungsten tip 5 is suppressed except from the front end.
In the thermal field emission electron gun of the construction shown in FIG. 2, the sintered zirconium 6' (Zr of ZrO.sub.2) coated on the tungsten tip 5 wears down with the lapse of time. This phenomenon is especially conspicuous at the interface between the tungsten tip 5 and the zirconium mass 6'. This interface is shown in the cross section of FIG. 3. That is, a gap 12 is created between the tungsten tip 5 and the zirconium mass 6'.
If this gap 12 forms and grows to about 10 .mu.m, the zirconium mass 6' slips off because the tungsten tip 5 is cylindrical in shape. This stops the generation of the electron beam from the tungsten tip 5. This slip occurs earlier than the wear of the zirconium itself and thus determines the life of this kind of thermal field emission electron gun.